The present invention relates to an optical disc molding apparatus used to mold a base of a disc such as an audio disc (CD (compact disc)), a video disc or the like, and more particularly relates to an optical disc molding apparatus in which an outer peripheral portion of a disk base may be protected from being deformed into a trumpet shape by improving a shape of a ring forming the outer peripheral portion of the disc base, resulting in a disc base of a high quality being obtained.
Heretofore, as optical recording media such as an audio disc, a video disc and so on for recording a variety of information, there are known optical recording media (hereinafter collectively referred to as "optical disc") for recording information by irradiation of recording or reproducing light. Such an optical disk generally includes an information recording layer. As a method of forming an information recording layer having very fine concavities and convexities such as phase bits, pregrooves and so on in which a variety of information such as data information, a tracking servo signal and so on is recorded, there has hitherto been used an injection molding method using a stamper.
FIG. 1 of the accompanying drawings is a cross-sectional view illustrating the mold clamping state of a metal molding apparatus for molding a disc base of an optical disc. In FIG. 1, reference numeral 1 generally denotes a stationary side die. In this stationary die 1, there is assembled a stationary metal mold 2 the side of which opposes a cavity is mirror-polished. Reference numeral 3 denotes a movable die which may be moved up and down relative to the stationary die 1 along a stay, not shown. Similarly, a movable metal mold 4 the side of which opposes the cavity is mirror-polished is assembled into the movable die 3. Cooling water paths 2a, 4a are defined in the stationary metal mold 2 and the movable metal mold 4, respectively,
A stamper 5 is disposed on the cavity surface side of the stationary metal mold 2 to transfer and mold a signal pit on the disc. The stamper 5 is held by a ring 6 which is served as also a metal mold for molding an outer peripheral portion of the disc.
More specifically, the ring 6 is formed such that the outer peripheral portion of the stamper 5 is urged against the cavity surface side of the stationary metal mold 2 by a land portion 8 of a convex shape formed on the upper edge of the innermost peripheral portion of an inner peripheral portion 7 of the ring 6. A thick outer peripheral portion 9 of the ring 6 is fixed to the stationary metal mold 2 by a plurality of bolts 10.
The movable die 3 is fitted into the ring 6 in such a manner that an outer peripheral surface of the movable metal mold 4 is brought in contact with the inner peripheral surface of the ring 6. Thus, the stationary metal mold 2, the movable metal mold 4 and the ring 6 constitute a molding cavity 11.
In this clamping state, as shown in FIG. 2 in an enlarged-scale, the land portion 8 of the inner peripheral portion 7 and the stamper 5 have a clearance C1 therebetween, the outer peripheral surface of the outer peripheral portion 9 and the stationary side die 1 have a clearance C2 therebetween, and the inner peripheral portion 7 and the movable metal mold 4 have a clearance C3 therebetween, respectively. The clearances Cl and C3 are set to be such clearance values as to pass only a gas within the cavity 11 but to inhibit a molten resin filled into the cavity 11 from entering, e.g. tens or more micrometers. The clearance C2 is set to be several micrometers, for example.
On the other hand, as shown in FIG. 1, the stationary side metal mold 2 has at its center attached a sprue 13 through a sprue bush 12 from the side of the stationary die 1. This sprue 13 has at its center defined an injection orifice 14 to inject a molten resin into the above-mentioned cavity 11. In FIG. 1, reference numeral 15 designates a nozzle guide for positioning an injection nozzle (not shown) against the sprue 13.
Moreover, the movable metal mold 4 has at its center attached a sleeve 17 through a bush 16 from the side of the movable die 3. A gate-cut piston 18 is disposed within the sleeve 17.
A manner in which an optical disc is molded by this prior-art optical disc molding apparatus will be described next.
Under the condition that the injection nozzle (not shown) is positioned at the sprue 13, a molten resin is injected from the injection nozzle through the injection orifice 14 into the cavity 11 and thereby filled into the cavity 11. As the molten resin is filled into the cavity 11, a gas which is in the gaseous state within the cavity 11 is escaped from a clearance (portion of the clearance C1) formed between the ring 6 and the stamper 5 and a clearance (portion of the clearance C3) formed between the ring 6 and the movable metal mold 4 so that the outer peripheral shape of the disc is finally molded by the ring 6. Then, after a predetermined time elapsed since the filling of the molten resin has been completed, the gate-cut piston 18 is elevated so that, at the same time the gate of the injection orifice 14 is cut out, a disc center-hole is molded by the outer peripheral portion of the gate-cut piston 18.
After the disc is cooled and cured, the movable die 3 is lowered together with the movable metal mold 4 and a molded disc base is obtained.
A molten resin 19 injected into the cavity 11 is filled into the cavity 11 at a high temperature (about 330.degree. C.) with a large pressure of 1000 kg/cm.sup.2 (1 ton). Due to such large filling pressure, the ring 6 which molds the outer peripheral portion of the disc base is swollen toward the outer diameter side by the amount of the clearance C2 as shown by a phantom line in FIG. 2, and in particular, the inner peripheral portion 7 of the ring 6 is buckled and deformed toward the stamper 5 side.
A stress produced in the ring 6 is a stress obtained when the ring 6 is returned to the original position after the resin pressure decreases as the resin is cooled and cured since the filling of the molten resin has been completed. A study of simulation results obtained by the assignee of the present application reveals that swollen portions 20a, 20b of a trumpet shape are produced on the upper and lower surfaces of the outer peripheral edge portion of a disc base 20 by the above stress as shown in FIG. 3. The swollen portion of the trumpet shape on the upper surface of the disc base 20, i.e. the swollen portion 20a on the signal pit side is swollen higher than the swollen portion 20b on the read-out side of the lower surface of the disc base 20.
As one of methods for preventing the ring 6 from being buckled, there may be considered a method of holding the'stamper 5 with the whole surface of the inner peripheral portion 7 of the ring 6. According to this method, although the buckled and deformed state of the ring 6 due to the filling pressure may be avoided, the outer peripheral portion of the stamper 5 is pressed down completely. There is then the problem that the stamper 5 is curved toward the cavity 11 side by an amount obtained when the stamper 5 is thermally expanded by the molten resin which is at a high temperature upon injection.
Further, there is a disadvantage that a burr tends to be produced in the outer peripheral portion of the molded disc base by a stress obtained when the ring 6 is returned to the original position.
Furthermore, there arises a problem that optical of optical disc are deteriorated by a birefringence produced in the outer peripheral portion of the molded disc due to the stress obtained after the ring 6 was returned to the original position.